1. Field of the Invention
The present invention relates generally to telephony. More specifically, systems and methods for reducing squealing or feedback on amplified telephones are disclosed.
2. Description of Related Art
An amplified telephone is a high performance telephone that amplifies audio signals to allow hearing impaired users to use the telephone without other hearing aides. When delivering a large amount of gain, the amplified telephone tends to suffer from squealing (i.e., feedback or the Larsen effect) as a result of the amplified telephone also amplifying background noise on the line such that the telephone's microphone (transmitter) picks up the amplified background noise. The signal generated by the microphone is again amplified through the amplified telephone's speaker (receiver) via side tone and the amplified telephone system becomes unstable. Side tone generally refers to the sound coming from the receiver that originated in the microphone. In other words, when a telephone user speaks into the telephone microphone, the user hears his own voice (side tone) through the receiver. Because of side tone, an amplified telephone generally cannot be amplified above a stability level of approximately 30 to 35 dB of gain without some mechanism for alleviating or preventing the resultant squealing. The squealing or Larsen effect is also commonly referred to as howling or acoustic instability similar to what happens when a microphone on a public address system is placed too close to a speaker.
Various methods have been implemented to alleviate or prevent the squealing in order to amplify the telephone above the 30 to 35 dB threshold gain in a stable manner. For example, a digital signal processor (DSP) may be employed to eliminate acoustic and line echoes. However, the cost of a DSP solution prevents its use in lower cost amplified telephones.
Another method to alleviate the squealing is to utilize half duplex, i.e., the telephone is either sending or receiving a signal but not both at the same time. In other words, a user using a half duplex telephone cannot simultaneously speak and hear what the other party is saying. Many speakerphones are half duplex. In half duplex mode, acoustic instability is controlled by muting or severely attenuating the receiver when the user is talking, and by muting or severely attenuating the transmitter when the user is listening. Half duplex produces the undesirable effect of the user having to wait until the person on the other end of the conversation stops speaking before the user can speak. Systems operating in half duplex mode are not as popular as full duplex systems and are much more difficult for hearing impaired persons to use. In addition, even sophisticated half duplex algorithms tend to produce an annoying artifact whereby the user notices that the phone is in half duplex.
Thus it would be desirable to provide an improved system and method for reducing the squealing or feedback on amplified telephones while minimizing or reducing undesirable artifacts.